The invention relates generally to radio wave antennae, and more particularly to multiple phase center radio wave antennae.
Multiple phase center antennae are used in some specialized communications and radar applications. Specific radar applications may include ground or airborne moving target indication (MTI), along with track interferometry and maritime surveillance. In MTI systems it may become difficult to discern a target from stationary background clutter when the target is moving slowly with respect to the terrain. Clutter is the term used in radar applications, to describe confusing or unwanted reflections that interfere with the observation of desired signals on a radar indicator. Clutter may be suppressed by receiving reflected radiation beams via multiple radar channels and employing adaptive filtering to identify stationary clutter from the moving target.
A multiple channel radar receiver may be implemented using multiple antennae, each antenna typically comprising a separate reflector excited by a feedhorn. This approach has several disadvantages, one being that the antenna directivity is limited to that of each individual antenna and not that implied by the physical span of the collective multiple antennae. Another disadvantage is that the phase center separation is mechanically fixed which also fixes the constant phase beamwidths. Finally, the system noise temperature increases linearly with the number of mismatched antenna apertures.
FIG. 1. shows an alternative approach where a single reflector antenna 100 is coupled to two feedhorns 102 and 104. feedhorns 102 and 104 are inclined at an angle to the centerline 106 of the reflector 100 thus establishing a pair of separated phase centers 110 and 112 at the antenna aperture 114. The separation increases with inclination angle of the feedhorns 102 and 104 to centerline 106.
The antenna configuration shown in FIG. 1 also suffers from several disadvantages. For maximum gain, the phase center of each of the feedhorns 102 and 104 should be at the focus 108 of reflector 100, but this is obviously impossible and hence a loss of antenna gain in the resulting radiation beam patterns must be suffered. Where more than two phase centers are required the problem is further exaggerated. Another disadvantage is that close placement of the feedhorns 102 and 104 commonly results in mutual coupling which may affect receiver discrimination. Furthermore, since MTI relies to a great extent on channel homogeneity the, the driving network for the feedhorns becomes increasingly complex requiring the provision of facilities for the calibration of the multiple beams. Yet another disadvantage is that, again, the phase center separation can only be changed by mechanical means. Furthermore for radar antenna that require rotation at high angular velocity, the added mass and pointing stability may also become an issue.
Accordingly there is a need for an antenna system that mitigates some of the above disadvantages.